1. Field of Invention
This invention relates to an optical system employing an illumination source with astigmatism, a collector lens and a beam expander, and more particularly to compensating for the astigmatism without adding optical components.
2. Description of the Prior Art
Optical record/playback systems are known. They include an illumination source, such as a laser diode, a collector lens to collimate the light from the illumination source, and an anamorphic beam expander to compensate for ovality in the output of the illumination source. The light beam output from the beam expander may then be directed toward the information surface of a suitable information bearing medium for playing (reading) or recording (writing) purposes, as described in U.S. Pat. No. 4,768,184, issued to the instant inventor and assigned to the same assignee, which is incorporated by reference herein.
Unfortunately, the output of a typical laser diode is not only not circular in cross-section but the apparent source points of the light are different as measured in two orthogonal planes in which lie two orthogonal axes, respectively, each orthogonal to the output direction. If the output direction from the laser diode is along the Z axis, the two orthogonal axes are typically termed the X axis and Y axis and the X-Z plane intercepts the X and Z axes while the Y-Z plane intercepts the Y and Z axes. The collector lens is typically positioned at its focal point distance from the source of the light to be collimated. Unfortunately, since there are two apparent source points at the two orthogonal planes and apparent source points at all points in between those planes, the question becomes where to put the collector. A common solution is to place the collector such that its focal point is coincident with one source point, say that along the X axis or Y axis of the laser diode and accept the resulting astigmatism. Another prior art solution is to add additional one or more cylinder lenses beyond the beam expander prisms but that adds additional cost, weight, and complexity.